Abstract:

A sliding member having a sliding surface comprising a silicon carbide
sintered body having a primary phase comprising mainly silicon carbide,
and a subphase having a different composition from the primary phase and
containing at least boron, silicon and carbon. The ratio of pores having
a roundness of 6 μm or less and a pore diameter of 10 to 60 μm with
respect to all pores having a pore diameter of 10 μm or more in the
sliding surface is 60% or more. This enables retention of good seal
properties even in a long-term continuous use. The subphase in the
silicon carbide sintered body is preferably granular crystal phases
dotted among a plurality of the primary phases. This provides excellent
lubricating liquid holding performance as well as excellent thermal
conductivity and excellent thermal shock resistance properties.

Claims:

1. A sliding member having a sliding surface comprising a silicon carbide
sintered body having a primary phase and a subphase, wherein the primary
phase comprises silicon carbide as a main component, and the subphase has
a different composition from the primary phase and contains boron,
silicon and carbon, whereinthe ratio of pores having a roundness of 6
μm or less and a pore diameter of 10 to 60 μm with respect to all
pores having a pore diameter of 10 μm or more in the sliding surface
is 60% or more.

2. The sliding member according to claim 1, wherein the ratio of the pores
in the sliding surface is 75% or more.

3. The sliding member according to claim 1, wherein the dispersion density
of the pores in the sliding surface is 60 pieces/mm2 or more.

4. The sliding member according to claim 1, wherein the porosity of the
silicon carbide sintered body is 2.5% to 12%.

5. The sliding member according to claim 1, wherein the maximum diameter
of each pore in the sliding surface is 100 μm or less.

6. The sliding member according to claim 1, wherein the subphase in the
silicon carbide sintered body is smaller than the particle diameter of
the primary phase adjacent thereto.

7. The sliding member according to claim 1, wherein the subphase in the
silicon carbide sintered body is a granular crystal phase dotted among a
plurality of the primary phases.

8. The sliding member according to claim 7, wherein the aspect ratio of
the subphase is 2.5 or less (excluding zero).

9. The sliding member according to claim 7, wherein the content of the
boron is 0.2 to 0.3% by mass with respect to 100% by mass of the silicon
carbide sintered body.

10. A method of manufacturing a sliding member according to claim 1,
comprising the steps of:the blending step of obtaining a raw material by
adding and mixing a pore forming agent and a pore dispersing agent for
dispersing the pore forming agent to silicon carbide powder as a main
ingredient;the molding step of obtaining a molded body by obtaining a
molding raw material by adding binder to the raw material, and then
molding the molding raw material into a predetermined shape; andthe
sintering step of obtaining a silicon carbide sintered body by sintering
the molded body.

11. The method of manufacturing a sliding member according to claim 10,
wherein the pore dispersing agent is an anionic interface activating
agent.

12. A mechanical seal ring using the sliding member according to claim 1.

13. A mechanical seal using the mechanical seal ring according to claim
12.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a sliding member constructed of a
silicon carbide sintered body, such as mechanical seal rings used in the
mechanical seals (shaft sealing devices) of pumps for fish holding tanks,
automobile cooling water pumps, refrigerators and the like. The invention
also relates to a method of manufacturing the sliding member, and a
mechanical seal ring using the sliding member and a mechanical seal using
the mechanical seal ring.

BACKGROUND ART

[0002]A sliding member using a ceramic sintered body is being applied by
use of the wear resistance thereof to mechanical seal rings used in, for
example, the mechanical seal of fluid equipments. The mechanical seal is
one of shaft sealing devices used in the rotating parts of various types
of machines with the aim of a complete fluid sealing. The mechanical seal
ring is made up of a rotary ring that slidingly contacts the rotary parts
of the various types of machines and is movable in the axial direction in
accordance with the wear of a sliding surface, and a stationary ring that
does not move. The mechanical seal ring operates to restrict the fluid
leakage at the end face substantially vertical to the relative rotating
shaft.

[0003]As the mechanical seal ring, a carbon material, a cemented carbide,
a silicon carbide sintered body or an alumina sintered body is used
mainly. In the recent years, a (porous) silicon carbide sintered body is
often used which has a high hardness and high corrosion resistance and
also has a low coefficient of friction during sliding and excellent
smoothness.

[0004]Patent document 1 has proposed a porous silicon carbide sintered
body with uniformly dispersedly arranged independent pores having a mean
pore diameter of 10 to 40 μm and having a porosity of 3% to 10%. FIG.
7 is a microphotograph showing the pores existing in the porous silicon
carbide sintered body proposed by the patent document 1.

[0005]In a sliding member using the silicon carbide sintered body proposed
by the patent document 1, though the wear resistance thereof has been
somewhat improved, a pore forming agent, such as polystyrene, for forming
pores is added into the raw material powder thereof.

[0006]However, the pore forming agent is liable to aggregate. As seen in
FIG. 7, this leads to a high ratio of communicating pores in which a
plurality of pores are communicated with each other so as to form a long
slender shape having a large maximum diameter, resulting in a high ratio
of the communicating pores with respect to the pores contributing to the
improvement of sliding characteristics, and having a pore diameter of 10
μm or more. Therefore, a long-term continuous use of the sliding
member has caused the problem that seal properties may rapidly
deteriorate because stress concentrates during sliding at the periphery
of the contour forming the communicating pores, thus being susceptible to
degranulation.

[0008]An advantage of the present invention is to provide a sliding
member, such as a mechanical seal ring, capable of maintaining excellent
seal properties even in a continuous long-term use.

[0009]Other advantage of the present invention is to provide a sliding
member, such as a mechanical seal ring, having excellent lubricating
liquid holding performance, as well as excellent thermal conductivity and
excellent thermal shock resistance.

Technical Solution

[0010]The sliding member of the invention has a sliding surface comprising
a silicon carbide sintered body having a primary phase and a subphase,
wherein the primary phase comprises silicon carbide as a main component,
and the subphase has a different composition from the primary phase and
contains boron, silicon and carbon. The ratio of pores having a roundness
of 6 μm or less and a pore diameter of 10 to 60 μm with respect to
all pores having a pore diameter of 10 μm or more in the sliding
surface is 60% or more.

[0011]The subphase in the silicon carbide sintered body is preferably
granular crystal phases dotted among a plurality of the primary phases.

[0012]The "sliding surface" in the invention means a surface where sliding
members are opposed to each other and rub each other. Besides the sliding
surface in the initial state thereof, a surface newly developed due to
wear during sliding is also included. Like the sliding surface, the
portions other than the sliding surface may comprise a primary phase
comprising mainly silicon carbide, and a subphase containing boron,
silicon and carbon. There is no problem if the primary phase and the
subphase differ in composition.

Advantageous Effects

[0013]The sliding member of the invention has the sliding surface
constructed of the silicon carbide sintered body having the primary phase
composed mainly of silicon carbide, and the subphase having a different
composition from the primary phase and containing at least boron, silicon
and carbon. The ratio of pores having a roundness of 6 μm or less and
a pore diameter of 10 to 60 μm with respect to all pores having a pore
diameter of 10 μm or more in the sliding surface is 60% or more. This
diminishes extremely large pores that deteriorate seal properties, and
also diminishes communicating pores, thereby facilitating a long-term
retention of seal properties.

[0014]Especially, when the subphase in the silicon carbide sintered body
is the granular crystal phase dotted among a plurality of the primary
phases, the movement of phonons as the carrier of thermal conduction is
hardly restricted, thereby improving thermal conductivity and thermal
shock resistance. As a result, the heat generation due to friction can be
lowered to diminish the wear of the sliding surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1(a) is a schematic explanatory drawing showing a sliding
surface of a sliding member according to a first preferred embodiment of
the invention; FIG. 1(b) is an enlarged schematic explanatory drawing
showing spherical pores in the sliding surface; FIGS. 1(c) and 1(d) are
enlarged schematic explanatory drawings showing communicating pores in
the sliding surface, respectively;

[0016]FIGS. 2(a) and 2(b) are microphotographs showing the states of pores
of a silicon carbide sintered body in the sliding member according to the
first preferred embodiment of the invention, specifically showing the
case where the porosity is 6% and the case where the porosity is 10%,
respectively;

[0017]FIG. 3(a) is a partial sectional view showing a mechanical seal
using a mechanical seal ring according to the first preferred embodiment
of the invention; FIG. 3(b) is a perspective view showing the mechanical
seal ring of FIG. 3(a);

[0018]FIG. 4(a) is a schematic explanatory drawing showing the crystal
structure of a silicon carbide sintered body in a sliding member
according to a second preferred embodiment of the invention; FIG. 4(b) is
an enlarged schematic explanatory drawing showing the subphase in FIG.
4(a);

[0019]FIG. 5 is a graph showing the EDS measurement result of the primary
phase of Sample No. I-1 in Example I;

[0020]FIG. 6 is a graph showing the EDS measurement result of the subphase
of Sample No. I-1 in Example I; and

[0022]A first preferred embodiment of the invention will be described
below in detail with reference to the accompanying drawings. FIG. 1(a) is
a schematic explanatory drawing showing the sliding surface of the
sliding member according to the first preferred embodiment of the
invention. FIG. 1(b) is an enlarged schematic explanatory drawing showing
spherical pores in the sliding surface. FIGS. 1(c) and 1(d) are enlarged
schematic explanatory drawings showing communicating pores in the sliding
surfaces, respectively. FIGS. 2(a) and 2(b) are microphotographs showing
the states of pores of a silicon carbide sintered body in the sliding
member according to the first preferred embodiment, specifically showing
the case where the porosity is 6% and the case where the porosity is 10%,
respectively. FIG. 3(a) is a partial sectional view showing a mechanical
seal using the mechanical seal ring according to the first preferred
embodiment. FIG. 3(b) is a perspective view showing the mechanical seal
ring of FIG. 3(a).

[0023]As shown in FIG. 1(a), the sliding member of the first preferred
embodiment has a sliding surface constructed of a silicon carbide
sintered body 1 having a primary phase 2 composed mainly of silicon
carbide, and a subphase 3 having a different composition from the primary
phase 2 and containing at least boron, silicon and carbon. Pores 4 exist
in the sliding surface.

[0024]The primary phase 2 means a phase in which the ratio (atomic %) of
silicon (Si) and carbon (C), namely Si:C is in the range of 35:65 to
65:35. The subphase 3 means a phase in which the ratio (atomic %) of Si
and C, namely Si:C is in the range of 0:100 to 34:66. The ratio (atomic
%) of Si and C is determined by the structural observation of the sliding
surface by a Transmission Electron Microscope (TEM), followed by an
energy dispersive X-ray spectroscopy analysis (EDS). Five locations are
measured and the average value thereof is used as the ratio of Si and C.
For example, in Sample No. I-1 in Example I described later, the EDS
measurement results of the primary phase is Si:C=44:56, as shown in FIG.
5, and the EDS measurement result of the subphase is Si:C=7:93, as shown
in FIG. 6.

[0025]The above-mentioned primary phase 2 and the subphase 3 can be
distinguished by color phases, respectively. That is, the primary phase 2
is a black phase, and the subphase 3 is a color phase exhibiting metallic
luster. These color phases can be distinguished by using, for example,
the backscattered electron image of the Scanning Electron Microscopy
(SEM).

[0026]Here, the subphase 3 has a different composition from the primary
phase 2 and contains at least boron, silicon and carbon. For example,
these elements may exist alone, or silicon (Si) and boron (B) may combine
together and exist as silicide and silicon carbide, such as SiB4 and
SiB6. The subphase 3 is a granular phase existing only in the region
surrounded by a plurality of the primary phases 2. When the subphase 3 is
a columnar phase or a needle-shaped phase extending over a plurality of
the primary phases 2, the movement of phonons as the carrier of thermal
conduction is subject to large restriction. Hereat, the Non-Patent
Document 1 discloses neither the sintering temperature nor the sintering
time for obtaining a silicon carbide sintered body 40. Depending on the
sintering temperature and the sintering time, a subphase 42 in the
silicon carbide sintered body 40 has a columnar shape, and the movement
of phonons is hindered because the subphase 42 is contained at a high
ratio. It is therefore considered that the silicon carbide sintered body
40 has poor thermal conduction and insufficient thermal shock resistance.

[0027]In the first preferred embodiment, the subphase 3 is the granular
phase dotted among a plurality of the primary phases 2. Hence, the
movement of phonons is hardly restricted, thereby improving thermal
conductivity and thermal shock resistance. As a result, the heat
generation due to friction can be lowered to diminish the wear of the
sliding surface.

[0028]The granular phase and the columnar or needle-shaped phase can be
distinguished by an aspect ratio. Specifically, the aspect ratio of the
granular phase is from 1 to below 4, and the aspect ratio of the columnar
or needle-shaped phase is 4 or more.

[0029]The subphase 3 is preferably smaller than the particle diameter of
the primary phases 2 adjacent thereto. This enables reduction in the
restriction exerted on the movement of phonons by the subphase 3 having
low thermal conductivity, thereby improving thermal conductivity and
thermal shock resistance. As a result, the heat generated due to friction
can be lowered to diminish the wear of the sliding surface. It can be
determined whether or not the subphase 3 is smaller than the particle
diameter of the primary phase 2 adjacent thereto, by observing the
sliding surface by a Scanning Electron Microscope.

[0030]In the silicon carbide sintered body 1, the ratio of the primary
phase 2 is suitably 99 to 99.8% by volume, and the ratio of the subphase
3 is suitably 0.2 to 1% by volume. These ratios can be measured by using,
for example, fluorescent X-ray analysis method, ICP (inductively coupled
plasma) emission analysis method, or carbon analysis method.

[0031]The pores 4 include two types of pores, specifically, pores
remaining along the grain boundaries without disappearing in the
sintering step, namely remaining pores, and pores generated by the
burning or thermal decomposition of a pore forming agent due to heating,
namely thermally-generated pores. These two types of pores are
distinguished by the pore diameters of pores existing in the sliding
surface of the sliding member. The remaining pores have a pore diameter
of less than 10 μm, and the thermally-generated pores have a pore
diameter of 10 μm or more. The pore diameters can be calculated by
using equation (2) described later.

[0032]The remaining pores have a pore diameter of less than 10 μm,
hardly affecting sliding characteristics and seal properties. On the
other hand, the thermally-generated pores have a large pore diameter of
10 μm or more, and therefore the shape and the distribution of the
thermally-generated pores existing on the sliding surface may greatly
affect the sliding characteristics and seal properties of the sliding
member.

[0033]The thermally-generated pores are classified into spherical pores 4a
shown in FIG. 1(b) having a roundness of 6 μm or less and a pore
diameter of 10 to 60 μm, and communicating pores 4b shown in FIGS.
1(c) and 1(d) in which a plurality of pores communicate with each other
or aggregate together. From the microphotographs showing the pores
existing in the sliding surface shown in FIGS. 2(a) and 2(b), it can be
seen that the sliding surface of the first preferred embodiment has the
spherical pores 4a, the communicating pores 4b and the remaining pores
4c.

[0034]The spherical pores 4a are constructed mainly of independent pores
independently existing without communicating with other pores. The
spherical pores 4a are of a substantially circle when the sliding surface
is viewed from above.

[0035]In the sliding member of the first preferred embodiment, the ratio
of the spherical pores 4a having a roundness of 6 μm or more and a
pore diameter of 10 to 60 μm with respect to all pores (the
thermally-generated pores) having a pore diameter of 10 μm or more in
the sliding surface is 60% or more.

[0036]Specifying the ratio of the spherical pores 4a at 60% or more
enables retention of high seal properties of the sliding members opposed
to each other in the sliding surface, and also enables a high ratio of
the spherical pores 4a contributing to improvement of sliding
characteristics, thus achieving high sliding characteristics.

[0037]The roundness of the spherical pores 4a adjusted to 6 μm or less
aims at retaining high seal properties in the sliding surface. That is,
the roundness of 6 μm or less prevents the lubricating liquid from
leaking more than necessary, and makes it difficult to degranulate, thus
permitting high seal properties. The roundness can be calculated by using
equation (1) described later.

[0038]The pore diameter adjusted to 10 to 60 μm aims at achieving
compatibility of sliding characteristics and seal properties in the
sliding surface. Especially, the ratio of the spherical pores 4a is more
preferably 75% or more, thereby further enhancing the seal properties in
the sliding surfaces. Although it is most effective to adjust the ratio
of the spherical pores 4a to 100% in order to improve seal properties and
sliding characteristics, it is preferable to adjust the ratio thereof to
90% or less in terms of manufacturing costs and production efficiency.

[0039]On the other hand, a high ratio of the pores having a roundness
exceeding 6 μm and the pores having a pore diameter exceeding 60 μm
may deteriorate seal properties. A high ratio of the pores having a pore
diameter of less than 10 μm increases the pores not contributing to
the improvement of sliding characteristics, resulting in poor sliding
characteristics.

[0042]From the above equation (1), the roundness of a pore becomes smaller
as it becomes closer to a perfect circle, and the roundness is zero when
it becomes the perfect circle. A larger difference between the maximum
diameter and the minimum diameter of the pore produces a larger value.
The roundness expressed by equation (1), the pore diameter (φ)
expressed by equation (2), and the ratio of the spherical pores 4a can be
measured by observing by an industrial microscope a surface that is
obtained by polishing the surface to be the sliding surface of the
sliding member by using diamond abrasive grains having a mean particle
diameter of 3 μm.

[0043]More specifically, they can be measured by the industrial microscope
set at 100 times magnification, and extracting and analyzing five
locations in the sliding surface, each location having a measuring area
of 1235 μm×926 μm. The maximum value of a pore corresponds to
the diameter of the minimum circumscribed circle surrounding the pore,
and the minimum value of the pore corresponds to the diameter of the
maximum inscribed circle surrounded by the pore. Accordingly, as shown in
FIG. 1(b), the maximum value (a) of the pore diameter is a diameter "a"
of a minimum circumscribed circle C1 surrounding the pore (the spherical
pore 4a), and the minimum value (b) of the pore diameter is a diameter
"b" of a maximum inscribed circle C2 surrounded by the pore (the
spherical pore 4a).

[0044]At the 100 times magnification, at least one point of inflection 4d
is observed in the contour forming the communicating pore 4b, and no
point of inflection is observed in the contour forming the spherical pore
4a.

[0045]In the sliding member of the first preferred embodiment, the
dispersibility of the spherical pores 4a in the sliding surface also
affects seal properties. Higher dispersibility of the spherical pore 4a
produces higher seal properties. Lower dispersibility, namely higher
agglutinability produces lower seal properties. In the first preferred
embodiment, the dispersion density of the spherical pores 4a in the
sliding surface is preferably 60 pieces/mm2 or more. This enables a
properly dispersed state with little aggregation of pores on the sliding
surface, thereby further improving seal properties. That is, the
dispersed state of the spherical pores 4a on the sliding surface is
brought into the dispersed state capable of retaining high seal
properties. Particularly, the dispersion density is more preferably 95
pieces/mm2 or more. In the same manner as when measuring the ratio
of the spherical pores 4a in the sliding surface, the dispersion density
can be measured by extracting five locations in the sliding surface, each
location having a measuring area of 1235 μmm×926 μm, and
analyzing them at 100 times magnification by the industrial microscope.

[0046]When the sliding surface is viewed from above, the spherical pores
4a have such a shape that the contour portions forming the spaces of the
pores are preferably continuous curves.

[0047]The sliding member of the first preferred embodiment has different
seal properties depending on the maximum diameter of the pores in the
sliding surface. For example, in the spherical pore 4a as shown in FIG.
1(b), the maximum diameter of the pore corresponds to the maximum value
"a" of the pore diameter. In the communicating pore 4b as shown in FIG.
1(c), the maximum diameter is L1. In the aggregated communicating pore 4b
as shown in FIG. 1(d), the maximum diameter is L2. The minimum value of
the pore in the communicating pore 4b as shown in FIG. 1(c) is L3. The
minimum value of the pore in the communicating pore 4b as shown in FIG.
1(d) is L4.

[0048]The maximum diameters of these pores are preferably 100 μm or
less. This avoids the presence of communicating pores having such an
extreme shape as to deteriorate seal properties and high agglutinability,
and spherical pores having an extremely large diameter, thus further
enhancing seal properties.

[0049]The maximum pore diameter can also be measured by extracting five
locations in the sliding surface, each location having a measuring area
of 1235 μm×926 μm, and analyzing them at 100 times
magnification by the industrial microscope.

[0050]In the sliding member of the first preferred embodiments the
porosity of the silicon carbide sintered body 1 constituting the sliding
surface is preferably 2.5% to 12%. This imparts high lubricating liquid
holding performance, so that sliding characteristics can be improved to
facilitate retention of mechanical characteristics.

[0051]That is, the porosity of the silicon carbide sintered body 1 also
affects mechanical characteristics in addition to the seal properties of
the sliding member and sliding characteristics. A high porosity improves
sliding characteristics, whereas seal properties and mechanical
characteristics are deteriorated. On the other hand, a low porosity
improves the seal properties and the mechanical characteristics of the
sliding member, whereas sliding characteristics are deteriorated. The
porosity adjusted to 2.5% to 12% decreases the ratio that the pores
existing on the sliding surface communicate with the pores existing on
the surfaces other than the sliding surface. This prevents that the
lubricating liquid supplied to the sliding surface leaks outside through
the communicating pores. Hence, it is easy for the lubricating liquid
held within the pores to form a continuous fluid film on the sliding
surface, thereby achieving high sliding characteristics and high seal
properties required for mechanical seal rings or the like.

[0052]In particular, the porosity is more preferably 3% to 8%. The
porosity of the silicon carbide sintered body 1 can be measured according
to Archimedean method.

[0053]A method of manufacturing the sliding member according to the first
preferred embodiment will be described below.

[0054]The steps of obtaining the sliding member include the blending step,
the molding step and the sintering step. These steps are described
sequentially.

[0055]In the blending step, slurry (a raw material) is obtained by adding
and mixing boron carbide powder, a sintering additive such as phenol
resin, a pore forming agent, a pore dispersing agent for dispersing the
pore forming agent, and water, etc. into silicon carbide powder as a main
ingredient.

[0056]As the pore forming agent, suspension-polymerized non-crosslinked
resin beads formed from at least one of silicone beads, polystyrene,
phenol resin and acryl-styrene copolymer may be used. The compression
strengths of these resin beads are as low as 1.2 MPa or less, and they
are therefore easily plastically deformed in the pressure direction
during the molding step, thereby diminishing micro cracks that are liable
to occur as elastic recovery proceeds. The pore forming agent is
thermally decomposed or eliminated to form pores (thermally-generated
pores) capable of supplying the lubricant onto the sliding surface.

[0057]Here, the first preferred embodiment employs the pore dispersion
agent. That is, in order to obtain the spherical pores 4a having a
roundness of 6 μm or less and a pore diameter of 10 to 60 μm with
respect to all pores having a pore diameter of 10 μm or more in the
sliding surface of the obtained sliding member, it is suitable to use as
the pore forming agent which has a roundness of 4 μm or less and a
diameter of 12 to 75 μm. However, the pore forming agent is a
hydrophobic material and hence cannot be dispersed in the slurry with
water added thereto, thus being susceptible to aggregation. There is a
high possibility that the formed pores are connected to each other on the
sliding surface. Depending on the case, seal properties may be
deteriorated. It is therefore necessary to disperse the pore forming
agent, and add the pore dispersing agent that functions to disperse the
pore forming agent. The added pore dispersing agent adsorbs the pore
forming agent, so that the pore forming agent easily wets and permeates,
and the reaggregation of the pore forming agent is reduced, thereby
enabling the pore forming agent to disperse without aggregating into the
slurry. In this case, 0.1% by mass or more of the pore dispersing agent
may be added to 100% by mass of the pore forming agent. Consequently,
irrespective of the type of the pore forming agent, the pore forming
agent can be sufficiently and easily dispersed to improve manufacturing
efficiency.

[0058]Preferable examples of the pore dispersing agent include anionic
interface activating agents such as carboxylate, e.g., polycarboxylic
acid sodium, sulfonate, sulfate ester and phosphate ester. The anionic
interface activating agent is highly effective in allowing the pore
forming agent to wet and permeate into the slurry. The anionic interface
activating agent adsorbing the pore forming agent enables the pore
forming agent to easily wet and permeate into the slurry. The
reaggregation of the pore forming agent can be further reduced by the
charge repulsion of hydrophilic groups contained in the anionic interface
activating agent. It is therefore easy to sufficiently disperse the pore
forming agent in the slurry without aggregation. Although when
manufacturing the slurry by mixing the silicon carbide powder as a main
ingredient with water, the silicon carbide slurry is alkalized to reduce
the aggregation of the silicon carbide powder, the aggregation of the
silicon carbide powder and the pore forming agent can also be reduced
even in the alkaline slurry by using the anionic interface activating
agent. Thus, the dispersion of the pore forming agent in the slurry
increases the ratio that the pores in the sliding surface of the obtained
sliding member exist as the independent spherical pores 4a, and
diminishes extremely large pores that deteriorate seal properties,
thereby enabling the long-term retention of seal properties.

[0059]In the molding step, granules are obtained by adding and mixing a
molding binder into the obtained slurry, followed by spray drying. Then,
a molding raw material with some of the granules encased in the pore
forming agent is obtained.

[0060]The content of the pore dispersing agent may be increased to
increase the dispersion density of the spherical pores 4a in the sliding
surface of the obtained sliding member. For example, in order to attain
the dispersion density of 60 pieces/mm2 or more, 1% by mass or more
of the dispersing agent may be added to 100% by mass of the pore forming
agent.

[0061]Similarly, in order to attain the pore maximum diameter of 100 μm
or less in the sliding surface of the obtained sliding member, the
content of the pore dispersing agent may be increased, and the pore
forming agent having a small diameter may be used. As a specific example,
1% by mass or more of the dispersing agent may be added, and the pore
forming agent having a diameter of 40 μm or less may be used with
respect to 100% by mass of the pore forming agent.

[0062]The porosity of the silicon carbide sintered body 1 constituting the
sliding member may be adjusted by, for example, the ratio of the pore
forming agent. As specific examples, the porosity of 2.5% or more is
attained by adjusting the ratio of the pore forming agent to 1% by mass
or more, and the porosity of 12% or less is attained by adjusting the
ratio of the pore forming agent to 5% by mass or less with respect to
100% by mass of the mixed powder of silicon carbide and boron carbide,
respectively.

[0063]The granules of the molding raw material are charged into a
predetermined mold and molded at a molding pressure suitably selected
from the range of 49 to 147 MPa, thereby obtaining a molded body.

[0064]In the sintering step, the molded body is defatted in nitrogen
atmosphere at a temperature of 450° C. to 650° C. for a
hold time of 2 to 10 hours, resulting in a defatted body. The defatted
body is then put in a sintering furnace and held in reduced pressure
atmosphere of an inert gas at a temperature of 1800° C. to
2100° C. for a hold time of 3 to 5 hours to sinter, resulting in a
silicon carbide sintered body 1. Through the blending step, the molding
step and the sintering step, the pore forming agent is uniformly
dispersed to diminish the extremely large pores that deteriorate seal
properties, and also decrease the communicating pores, thus facilitating
a long-term retention of seal properties. Although no special limit is
imposed on the inert gas, argon gas is suitably used because it is easy
to purchase and handle.

[0065]The pressed surface of the obtained sintered body may be subjected
to grinding, polishing or the like when necessary. For example, the
sliding surface may be obtained by flattening the pressed surface with a
double-head grinding machine or a surface grinding machine, and roughly
machining it with a lapping machine made of alumina by using diamond
abrasive grains having a mean particle diameter of 3 μm, and then
mirror-finishing it with a lapping machine made of tin by using diamond
abrasive grains having a mean particle diameter of 1 μm so that the
arithmetic mean height Ra is 0.98 μm or less. The arithmetic mean
height Ra of 0.98 μm or less facilitates retention of seal properties.

[0066]The arithmetic mean height Ra may be measured according to JIS B
0601-2001 (corresponding to ISO 4287:1997). That is, when the measuring
length and the cut-off value are set at 5 mm and 0.8 mm, respectively,
and the measurement is carried out by using a contact surface roughness
tester, a stylus having a tip end radius of 2 μm may be applied to the
sliding surface of the sliding member, and the scanning rate of the
stylus may be set at 0.5 mm/sec.

[0067]The surface of the sintered body is thus polished to provide a
mechanical seal ring. According to the above manufacturing method, it is
inexpensive to obtain the sliding member such as the mechanical seal ring
having excellent lubricating liquid retention performance and excellent
thermal conductivity and thermal shock resistance.

[0068]The mechanical seal ring and the mechanical seal, each using the
sliding member according to the above-mentioned first preferred
embodiment will be described below.

[0069]As shown in FIG. 3(a), the mechanical seal is a device using a
mechanical seal ring 5 that exerts sealing action by sliding a sliding
surface 15b of the rotary ring 5b as an annular body having a convex
portion, on a sliding surface 15a of the stationary ring 5a as an annular
body. The mechanical seal ring 5 is mounted between a rotary shaft 6
transmitting the driving force supplied from a driving mechanism (not
shown) and a casing 7 rotatably supporting the rotary shaft 6, so that
the sliding surfaces 15a and 15b of the stationary ring 5a and the rotary
ring 5b form vertical surfaces to the rotary shaft 6, respectively.

[0070]The mechanical seal ring 5 consists of the stationary ring 5a and
the rotary ring 5b for bringing the sliding surfaces 15a and 15b into
contact and slide through a lubricating liquid, respectively. At least
one of the stationary ring 5a and the rotary ring 5b is formed from the
sliding member of the first preferred embodiment. The sliding member has
excellent seal properties and lubricating liquid retention performance as
described above. Consequently, the mechanical seal ring 5 and the
mechanical seal each using the sliding member have high long-term
reliability.

[0071]The rotary ring 5b is cushioningly supported by a packing 8. A coil
spring 9 is mounted so as to wind around the rotary shaft 6 on the side
opposed to the rotary ring 5b of the packing 8. By pressing the packing 8
with the springback force of the coil spring 9 (the preset force of the
coil spring 9), the sliding surface 15b of the rotary ring 5b is pressed
so as to cause sliding by the sliding surface 15a of the stationary ring
5a. A collar 10 is fixed by a set screw 11 to the rotary shaft 6 and
mounted as the stopper of the coil spring 9 on the opposite side from
that the coil spring 9 presses the packing 8.

[0072]The stationary ring 5a contacting the sliding surface 15b of the
rotary ring 5b through the sliding surface 15a is supported by a cushion
rubber 12. The cushion rubber 12 is mounted inside a casing 7, serving as
the outer frame of the mechanical seal, so as to support the stationary
ring 5a. When the rotary shaft 6 is rotated, the collar 10 is also
rotated. Then, the packing 8 pressed by the springback force of the coil
spring 9, and the sliding surface 15b of the rotary ring 5b supported by
the packing 8 are rotated while being pressed, thereby exerting the
sealing action with the sliding surface 15a of the stationary ring 5a.
When the mechanical seal is mounted on a fluid equipment (not shown), the
mechanical seal is mounted so that the fluid equipment is arranged on the
extension of the collar 10 with respect to the mechanical seal ring 5.

[0073]At this time, the fluid enters into the inside surrounded by the
casing 7 of the mechanical seal. However, the sealing action of an o-ring
13 mounted between the packing 8 and the rotary shaft 6, and the sealing
action of the sliding surfaces 15a and 15b of the mechanical seal ring 5
cooperate to eliminate the fluid leakage from the mechanical seal to the
outside. The fluid sealed by the mechanical seal at this time is called a
sealed fluid 14, part of which enters into between the sliding surfaces
15a and 15b of the mechanical seal ring 5 and acts as a lubricating
liquid. On the other hand, the rotary ring 5b is cushioningly supported
by the packing 8, and the cushion rubber 12 and the packing 8 also
function to absorb vibrations generated by the rotation of the rotary
shaft 6.

[0074]When the rotary ring 5b starts to slide, the dynamic pressure due to
air flow is firstly generated on the sliding surfaces 15a and 15b.
Subsequently, on the spherical pores 4a, negative pressure lower than the
dynamic pressure is applied to the lubricating liquid retained within the
spherical pores 4a. The negative pressure generated on the spherical
pores 4a enables the lubricating liquid retained within the spherical
pores 4a to be suitably supplied to the sliding surfaces 15a and 15b,
thereby providing the mechanical seal ring 5 having high strength and
high sliding characteristics.

[0075]In the mechanical seal shown in FIG. 3(a), the stationary ring 5a is
the annular body, and the rotary ring 5b is the annular body having the
convex portion. Conversely, the stationary ring 5a may be the annular
body having the convex portion, and the rotary ring 5b may be the annular
body.

[0076]A second preferred embodiment of the invention will next be
described in detail with reference to the accompanying drawings. FIG.
4(a) is a schematic explanatory drawing showing the crystal structure of
a silicon carbide sintered body in a sliding member according to the
second preferred embodiment. FIG. 4(b) is an enlarged schematic
explanatory drawing showing the subphase of FIG. 4(a).

[0077]As shown in FIG. 4(a), the sliding member of the second preferred
embodiment has a sliding surface formed from a silicon carbide sintered
body 16 having a primary phase 17 composed mainly of silicon carbide, and
a subphase 18 containing at least boron, silicon and carbon. The subphase
18 is a granular crystal phase dotted independently among a plurality of
the primary phases 17.

[0078]That is, the subphase 18 of the second preferred embodiment is the
granular phase existing only in the regions surrounded by the plurality
of the primary phases 17. When the subphase 18 is a columnar phase or a
needle-shaped phase extending over a plurality of the primary phases 17,
the movement of phonons as the carrier of thermal conduction is subject
to large restriction. In the second preferred embodiment, the subphase 18
is the granular phase dotted among a plurality of the primary phases 17,
the movement of phonons is hardly restricted, so that both thermal
conductivity and thermal shock resistance can be improved. As a result,
the heat generation due to friction can be lowered to diminish the wear
of the sliding surface.

[0079]Particularly, a distance "d" between the adjacent subphases 18 is
preferably 3 μm or more. Thereby, the movement of phonons is further
unsusceptible to restriction.

[0080]The state in which the subphase 18 is dotted among a plurality of
the primary phases 17, and the distance "d" can be confirmed by observing
the cross section of the silicon carbide sintered body 16 or the sliding
surface by a Transmission Electron Microscope or a scanning electron
microscope set at 3000 to 10000 times magnification.

[0081]In addition to boron, silicon and carbon, unavoidable impurities
such as sodium (Na), magnesium (Mg), iron (Fe), aluminum (Al) and calcium
(Ca) may be contained in the subphases 18, which will cause no problem.
From the viewpoint of maintaining mechanical characteristics, the total
amount of these unavoidable impurities is preferably 0.1% by volume or
less with respect to the silicon carbide sintered body 16.

[0082]The thermal conductivity and thermal shock resistance of the sliding
member is susceptible to the influence of the shape of the subphase 18,
namely the aspect ratio thereof. As shown in FIG. 4(b), the aspect ratio
of the subphase 18 is the ratio of a long axis β to a short axis
α (namely, the long axis β/the short axis α). The
movement of phonons is more unsusceptible to restriction as the ratio
becomes smaller, thereby improving the thermal conductivity and thermal
shock resistance of the sliding member.

[0083]In the second preferred embodiment, the aspect ratio of the subphase
18 is preferably 2.5 or less (excluding 0). Thereby, the movement of
phonons is more unsusceptible to restriction, enabling further
improvement in both the thermal conductivity and thermal shock resistance
of the sliding member. As a result, the heat generation due to friction
can be lowered, and the wear of the sliding surface can be further
diminished.

[0084]The aspect ratio of the subphase 18 can be measured from the image
of the cross section of the silicon carbide sintered body 16 or the
sliding surface by a Transmission Electron Microscope or a scanning
electron microscope set at 3000 to 10000 times magnification.

[0085]As described above, the subphase 18 contains at least boron, silicon
and carbon. As described later, in the method of manufacturing the
silicon carbide sintered body constituting the sliding member of the
invention, silicon and carbon in the subphase are obtained by molding and
sintering the raw material powder prepared by mixing boron carbide powder
and the like into silicon carbide powder. Therefore, the silicon and the
carbon within the silicon carbide sintered body are contained as the
subphase in the silicon carbide sintered body. Especially, the boron
contained in the subphase 18 performs an important action in the second
preferred embodiment and affects on the mechanical characteristics and
thermal conductivity of the sliding member. When the content of boron is
too low, the crystal particles of silicon carbide cannot be sufficiently
bonded together, thus deteriorating mechanical characteristics and
thermal conductivity. On the other hand, when the content of boron is too
high, a subphase having a high aspect ratio is deposited and the movement
of phonons is susceptible to restriction, thereby deteriorating thermal
conductivity. In the sliding member of the second preferred embodiment,
the content of boron is preferably 0.2 to 0.3% by mass with respect to
100% by mass of the silicon carbide sintered body. By adjusting the
content of boron to the above-mentioned range, the boron acts as
sintering additive, resulting in the sliding member having both high
mechanical characteristics and high thermal conductivity.

[0086]The content of boron can be measured by using fluorescent X-ray
analysis method or ICP emission analysis method. Most of the boron form
the subphase 18 together with silicon and carbon, and some boron may be
dispersed into the crystal particles of silicon carbide.

[0087]Also in the second preferred embodiment, the porosity of the silicon
carbide sintered body 16 is preferably 2.5% to 12%, more preferably 3% to
8%, for the same reason as described in the first preferred embodiment.

[0088]A method of manufacturing the sliding member according to the second
preferred embodiment will be described below.

[0089]Firstly, slurry is obtained by adding water, a dispersing agent,
boron carbide powder, a sintering additive such as and phenol resin, a
pore forming agent, a pore dispersing agent into silicon carbide powder,
and then mixing and grinding with a ball mill (the blending step).
Silicon carbide granules are obtained by adding and mixing binder into
the slurry, followed by spray drying. These granules are then molded to
obtain a molded body (the molding step).

[0090]The content of boron with respect to the silicon carbide sintered
body 16 is subjected to the influence of the added boron carbide powder.
In order to adjust the content of boron to 0.2 to 0.3% by mass with
respect to 100% by mass of the silicon carbide sintered body, the content
of boron carbide powder may be adjusted to 1 to 3% by mass with respect
to the silicon carbide powder.

[0091]In order to adjust the porosity of the silicon carbide sintered body
16 to 2.5% to 12%, 0.5 to 10% by mass of resin beads, which are
previously ground as a pore forming agent to be burned out or thermally
decomposed in the defatting step or the sintering step, is added to the
granules and mixed together to prepare a mixed raw material. The mixed
raw material is then charged into a mold and pressed and molded,
resulting in a molded body having a predetermined shape. Examples of the
above-mentioned resin beads include the same one as described in the
first preferred embodiment.

[0092]When necessary, the temperature of the obtained molded body may be
raised for 10 to 40 hours in nitrogen atmosphere, and maintained at
450° C. to 650° C. for 2 to 10 hours, then spontaneously
cooled and defatted. For example, the silicon carbide sintered body 16 is
obtained by holding the obtained defatted molded body in reduced pressure
atmosphere of an inert gas at a temperature of 1800° C. to
2100° C. for 3 to 5 hours to sinter (the sintering step).

[0093]The aspect ratio of the subphase 18 is especially susceptible to the
influence of the sintering temperature. Increasing the sintering
temperature leads to a large aspect ratio value, and decreasing the
sintering temperature leads to a small aspect ratio value. In order to
adjust the aspect ratio of the subphase 18 to 2.5 or less (excluding 0),
the sintering temperature may be adjusted to 1800° C. to
2000° C.

[0094]The distance "d" between the adjacent subphases 18 is susceptible to
the influence of the sintering time. Increasing the sintering time leads
to a large distance value, and decreasing the sintering time leads to a
small distance value. In order to adjust the distance "d" between the
adjacent subphases 18 to 3 μm or more, the sintering time may be in
the range of 4.5 to 5 hours.

[0095]According to the above manufacturing method, it is inexpensive to
attain the sliding member such as the mechanical seal ring having
excellent lubricating liquid retention performance as well as excellent
thermal conductivity and excellent thermal shock resistance.

[0096]The mechanical seal ring and the mechanical seal, each using the
sliding member according to the second preferred embodiment will be
described below. In the mechanical seal ring and the mechanical seal
according to the second preferred embodiment, at least one of the
stationary ring 5a and the rotary ring 5b is formed from the sliding
member according to the second preferred embodiment (refer to FIG. 3).

[0097]The sliding member has excellent thermal conductivity and excellent
thermal shock resistance. Therefore, the mechanical seal ring and the
mechanical seal according to the second preferred embodiment can be
suitably used under severe use conditions in which at the start of
sliding, high friction heat is momentarily generated, thus being
susceptible to thermal shock.

[0098]Otherwise, the configuration is identical to that of the first
preferred embodiment, and therefore the description thereof is omitted
here.

[0099]While the preferred embodiments of the present invention have been
described and illustrated above, it is to be understood that they are
exemplary of the invention and are not to be considered to be limiting.
Changes and modifications can be made thereto without departing from the
gist of the present invention. For example, the invention is not limited
to the sliding members according to the first and second preferred
embodiments, respectively. For example, the invention may be a sliding
member according to other preferred embodiment as a combination of the
sliding member of the first preferred embodiment and the sliding member
of the second preferred embodiment.

[0100]The pore shape in the invention may be a columnar shape instead of
the spherical pores 4a, as long as the roundness is 6 m or less and the
pore diameter is 10 to 60 μm in the sliding surfaces.

[0101]The invention will be described below in detail based on examples.
However, it is to be understood that the invention is not limited to the
following examples.

EXAMPLE I

<Sample Preparation>

[0102]A predetermined amount of boron carbide powder was added to silicon
carbide powder as a main ingredient, and suspension-polymerized
non-crosslinked resin beads composed of phenol resin and polystyrene
having the maximum diameter shown in Table 1 was added thereto as a pore
forming agent. The pore forming agent having a roundness of 4 μm or
less and a diameter of 12 to 75 μm was used in each sample. The pore
forming agent was added at the ratio shown in Table 1, with respect to
100% by mass of the mixed powder of silicon carbide and boron carbide.
Further, as a pore dispersing agent, polycarboxylic acid sodium was added
and mixed at the ratio shown in Table 1, with respect to 100% by mass of
the pore forming agent, thereby obtaining a raw material.

[0103]The obtained raw material was put into a ball mill and mixed for 48
hours to make slurry. As a molding additive, binder was added and mixed
to the slurry, followed by spray drying, thereby obtaining a molding raw
material composed of silicon carbide granules having a mean particle
diameter of 80 μm.

[0104]The molding raw material was then charged into a mold and pressed
and molded at a pressure of 98 MPa in the thickness direction, thereby
obtaining a ring-shaped molded body. The temperature of the obtained
molded body was raised for 20 hours in nitrogen atmosphere, and held at
600° C. for 5 hours, then spontaneously cooled and defatted,
thereby obtaining a defatted body.

[0105]Finally, the defatted body was held at about 2000° C. for 4
hours and then sintered to manufacture a silicon carbide sintered body
having a primary phase of silicon carbide, and a subphase containing
boron, silicon and carbon.

[0106]The surface of each of the obtained silicon carbide sintered bodies
was ground by a surface grinding machine, and roughly machined by a
lapping machine made of alumina using diamond abrasive grains having a
mean particle diameter of 3 μm. Subsequently, the surface thereof was
polished by a lapping machine made of tin using the diamond abrasive
grains having a mean particle diameter of 3 μm, so as to have an
arithmetic mean height (Ra) of 0.98 μm or less, thereby obtaining a
sliding surface. Thus, Sample Nos. I-1 to I-18 were manufactured which
were mechanical seal rings having an outer diameter of 26 mm and an inner
diameter of 19 mm. These samples were the stationary rings 5a as shown in
FIG. 3.

[0107]Setting an industrial microscope at 100 times magnification, five
locations, each location having a measuring area of 1235 μm×926
μm in the sliding surface of each sample thus obtained, were extracted
and analyzed to measure the ratio of spherical pores having a roundness
of 6 μm or less and a pore diameter of 10 to 60 μm, the dispersion
density of these spherical pores, and the maximum diameter of the pores
in the individual sliding surface. At this time, the area ratios of the
primary phase and the subphase were also measured. As a result, the
primary phase accounted for 95% by area, and the subphase accounted for
5% by area with respect to 100% by mass of the total area of the primary
phase and the subphase.

[0108]When measuring the ratio (atomic %) of Si and C in the primary phase
and that of the subphase, the structural observation on the sliding
surface was carried out by a TEM, followed by measurement by an Energy
Dispersive X-ray Spectroscopy Analysis (EDS). Specifically, five
locations were measured, and the average value thereof was used as the
ratio of Si and C. It was evaluated whether the ratio of Si and C, Si:C,
in the primary phase was within the range of 35:65 to 65:35, and the
ratio of Si and C, Si:C, in the subphase was within the range of 0:100 to
34:66. As an example of the measurement results by the Energy Dispersive
X-ray Spectroscopy Analysis (EDS), the measurement result of the primary
phase in Sample No. I-1 is shown in FIG. 5, and the measurement result of
the subphase is shown in FIG. 6. The primary phase in Sample No. I-1 had
the result of Si:C=44:56, and the subphase had the result of Si:C=7:93.

[0109]The porosity of the silicon carbide sintered body constituting each
sample was measured according to Archimedean method.

<Characteristic Evaluation>

[0110]There was prepared a rotary ring 5b composed of carbon which was an
annular body having an outer diameter of 26 mm and an inner diameter of
19 mm, and had a convex portion having an outer diameter of 24 mm and an
inner diameter of 21 mm. The rotary ring 5b and each of the stationary
rings 5a (Sample Nos. I-1 to I-18) were brought into contact with sliding
surfaces 15a and 15b through a rotary shaft 6, and then slid under the
following sliding conditions to measure the leak amount from the sliding
surfaces 15a and 15b, indicating seal properties, and measure the
coefficient of friction indicating sliding characteristics.

<Sliding Conditions>

[0111]Relative speed: 8 m/s

[0112]Surface pressure: 500 kPa

[0113]Lubricating liquid: water

[0114]Sliding time: 100 hours

[0115]The relative speed is the rotation speed of the rotary ring 5b with
respect to the stationary ring 5a at a position facing to the outer
periphery with reference to the center of the rotary shaft, and being
spaced therefrom by 11.25 mm (hereinafter referred to as position P). The
surface pressure is the pressure per unit area of the rotary ring 5b with
respect to the stationary ring 5a, and is found by dividing a
pressurizing force F preset for bringing the stationary ring 5a and the
rotary ring 5b into contact with each other, by the area of the sliding
surface 15b of the rotary ring 5b. Setting a metal microscope provided
with a gauge at 50 times magnification, the area was calculated by
measuring with the gauge the outer diameter and the inner diameter of the
convex portion of the rotary ring 5b.

[0116]With regard to the coefficient of friction "μ", the rotation
torque T at the position P of the rotary ring 5b during sliding was
measured by using a torque meter. The pressuring force F was obtained by
multiplying the area of the sliding surface 15b by a surface pressure.
Then, the coefficient of friction "μ" was obtained by dividing the
rotation torque T by the pressuring force F and the distance 11.25 mm
from the center of the rotary shaft to the position P. That is, the
coefficient of friction "μ" was a value calculated from the following
equation: μ=T/11.25F. The obtained values are shown in Table 1.

[0118]In Table 1, the ratio of spherical pores having a roundness of 6
μm or less and a pore diameter of 10 to 60 μm to all pores having a
pore diameter of 10 μm or more in the sliding surfaces, and the
dispersion density of the spherical pores are represented as merely the
ratio of the spherical pores and the dispersion density of the spherical
pores for simplicity.

[0119]As apparent from Table 1, in the samples of the invention (Nos. I-4
to I-20) in which the ratio of the spherical pores having a roundness of
6 μm or less and a pore diameter of 10 to 60 μm with respect to all
pores having a pore diameter of 10 μm or more was 60% or more, the
amount of leak from between the sliding surfaces 15a and 15b was as small
as 120 ml or less and exhibited high seal properties than the samples
(Nos. I-1 to I-3) in which the ratio of the spherical pores was less than
60%. Especially, in the samples (Nos. I-7 to I-19) in which the ratio of
the spherical pores was 75%, the leak amount in the sliding surfaces was
65 ml or less, exhibiting higher seal properties.

[0120]The following results were obtained when comparing the samples in
which the dispersion density of the spherical pores, the maximum diameter
of the pores, and the porosity of the silicon carbide sintered body were
changed in the samples (Nos. I-7 to I-19) in which the ratio of the
spherical pores was 75%.

[0121]That is, it can be seen that in each of the samples (Nos. I-8 to
I-19) in which the dispersion density of the spherical pores in the
sliding surfaces is 60 pieces/mm2 or more, the leak amount is as
small as 54 ml or less, exhibiting higher seal properties than the sample
(No. I-7) in which the dispersion density is less than 60
pieces/mm2. When comparing the samples (Nos. I-8 to I-19) in which
the dispersion density of the spherical pores in the sliding surfaces is
60 pieces/mm2 or more, it can be seen that the samples (Nos. I-8 and
I-10 to I-19) in which the maximum diameter of the individual pores is
100 μm or less, the leak amount is as small as 35 ml or less,
exhibiting higher seal properties than the sample (No. I-9) in which the
maximum diameter of the individual pores exceeds 100 μm.

[0122]In the samples (Nos. I-8 to I-11, I-13 to I-16, I-18 and I-19) in
which the porosity of the silicon carbide sintered body is 2.5% to 12%,
the coefficient of friction was as low as 0.04 or less, and the
four-point bending strength was as high as 201 MPa or more. On the other
hand, in the sample (No. I-12) in which the porosity was less than 2.5%,
the coefficient of friction was as high as 0.08. In the sample (No. I-17)
in which the porosity exceeded 12%, the coefficient of friction was low
whereas the four-point bending strength was as low as 190 MPa.

EXAMPLE II

<Sample Preparation>

[0123]Firstly, boron carbide powder, a pore forming agent, a pore
dispersing agent and water, each having the amount of addition shown in
Table 2, were added to silicon carbide powder, and put into a ball mill
and mixed for 48 hours to make slurry. As a molding additive, binder was
added and mixed to the slurry, followed by spray drying, thereby
obtaining silicon carbide granules having a mean particle diameter of 80
μm.

[0124]As the above pore forming agent, suspension-polymerized
non-crosslinked resin beads composed of previously ground polystyrene
were used, which had a roundness of 4 μm or less and a diameter of 12
to 75 μm, and had the maximum diameter shown in Table 2. As the above
pore dispersing agent, polycarboxylic acid sodium was used.

[0125]Subsequently, the mixed raw material was charged into a mold, and
then pressed and molded at a pressure of 98 MPa in the thickness
direction, thereby obtaining a molded body having a predetermined shape.
The temperature of the obtained molded body was raised for 20 hours in
nitrogen atmosphere, and held at 600° C. for 5 hours, then
spontaneously cooled and defatted, thereby obtaining a defatted body.

[0126]The defatted body thus obtained was then held at the sintering
temperature shown in Table 2 for 4 hours, thereby manufacturing Sample
Nos. II-1 to II-10, each of which was a silicon carbide sintered body
having a primary phase of silicon carbide and a subphase containing
boron, silicon and carbon.

[0127]The content of boron with respect to 100% by mass of the sintered
body of each sample was measured by ICP emission analysis method. The
measured values are shown in Table 2. In present example, every boron was
contained in the subphase.

[0128]The surface of each sample was ground to obtain a flat surface by a
surface grinding machine, and roughly machined by a lapping machine made
of alumina using diamond abrasive grains having a mean particle diameter
of 3 μm. Subsequently, the surface thereof was mirror-finished by a
lapping machine made of tin using the diamond abrasive grains having a
mean particle diameter of 3 μm, so as to have an arithmetic mean
height Ra of 0.98 μm or less, thereby obtaining a sliding surface. The
subphase shape and the aspect ratio in the sliding surface were observed
and measured at 5000 times magnification by a Scanning Electron
Microscope. The measured values are shown in Table 2.

[0129]The ratio of spherical pores, and the ratios of Si and C (atomic %)
in the primary phase and that of the subphase were measured in the same
manner as in Example 1. The measured values and the measuring results are
shown in Table 2.

<Evaluation>

[0130]The three-point bending strength, the Poisson's ratio, the Young's
modulus, the coefficient of thermal expansion at 40° C. to
400° C. and the thermal conductivity in each sample were measured
separately. Specifically, the three-point bending strength (S) was
measured according to JIS R 1601-1995 (corresponding to ISO 14704: 2000
or ICS 81.060.30). The Poisson's ratio (ν) and the Young's modulus (E)
were measured according to JIS R 1602-1995 (corresponding to ISO 17561:
2002). The coefficient of thermal expansion (α) at 40° C. to
400° C. was measured according to JIS R 1618-2002 (corresponding
to ISO 17562: 2001). The thermal conductivity (k) was measured according
to JIS R 1611-1997.

[0131]The coefficient of thermal shock resistance R was calculated by
applying the three-point bonding strength (S), the Poisson's ratio
(ν), the Young's modulus (E) and the coefficient of thermal expansion
(α) at 40° C. to 400° C. thus obtained by the above
measurements, to the following equation (3). Then, the coefficient of
thermal shock resistance R' was calculated by applying the coefficient of
thermal shock resistance R thus calculated and the thermal conductivity
(k) thus obtained by the above measurement, to the following equation
(4).

[Equation 3]

R=S×(1-ν)/(E×α) (3)

where S is a three-point bending strength (Pa), "ν" is a Poisson's
ratio, E is a Young's modulus (Pa), and "α" is a coefficient of
thermal expansion at 40° C. to 400° C.
(×10-6/K).

[Equation 4]

R'=R×k (4)

where "k" is a thermal conductivity (W/(mK)).

[0132]Here, the coefficient of thermal shock resistance R is a coefficient
that becomes an index of thermal shock resistance properties when heated
and quickly cooled. The coefficient of thermal shock resistance R' is a
coefficient that becomes an index of thermal shock resistance properties
when heated and relatively gently cooled. It can be said that higher
thermal shock resistance properties are attainable when these
coefficients have higher values.

[0133]The measurement results of the thermal conductivity k and the
coefficient of thermal shock resistance R' are as shown in Table 2.

[0134]The porosity of the silicon carbide sintered body constituting each
sample was measured according to Archimedean method.

[0135]Separately, a ring-shaped molded body was manufactured, and then
defatted and sintered, thereby obtaining a sintered body. The surface
thereof was ground to obtain a flat surface by a surface grinding
machine, and roughly machined by a lapping machine made of alumina.
Subsequently, the surface thereof was mirror-finished by a lapping
machine made of tin so as to have an arithmetic mean height Ra of 0.98
μm or less, thereby obtaining sample Nos. II-1 to II-10, each being an
annular body having an outer diameter of 26 mm and an inner diameter of
19 mm. All of these samples were stationary rings 5a.

[0136]Subsequently, each of the rotary rings 5b prepared in Example I and
each of the stationary rings 5a thus obtained (Sample Nos. II-1 to II-10)
were brought into contact with the sliding surfaces 15a and 15b through
the rotary shaft 6, and then slid under the same sliding conditions as in
Example I, thereby measuring the coefficient of friction. The measured
values are shown in Table 2.

[0137]As apparent from Table 2, in Sample Nos. II-2 to II-10 of the
invention, the subphase thereof was granular and had a high aspect ratio,
and hence they had a high thermal conductivity and a high coefficient of
thermal shock resistance R', exhibiting high thermal conductivity and
high thermal shock resistance properties.

[0138]Particularly, Sample Nos. II-3 to II-7, II-9 and II-10, in which the
aspect ratio was 2.5 or less, had a higher thermal conductivity and a
higher coefficient of thermal shock resistance R'.

[0139]When comparison among Sample Nos. II-4 to 11-8 having different
contents of boron was made, Sample Nos. II-5 to II-7, in which the
content of boron was 0.2 to 0.3% by mass, had a higher thermal
conductivity and a higher coefficient of thermal shock resistance R' than
Sample Nos. II-4 and II-8, in which the content of boron was outside the
above-mentioned range.

[0140]On the other hand, in Sample No. II-1 being outside the scope of the
invention, a different phase not being the composition of the subphase of
the invention was generated, exhibiting a columnar shape. Since the
different phase had a higher aspect ratio, this sample had a low thermal
conductivity and a low coefficient of thermal shock resistance R',
exhibiting low thermal conductivity and low thermal shock resistance
properties.

EXAMPLE III

<Sample Preparation>

[0141]Firstly, 2.5% by mass of boron carbide powder, a pore forming agent,
a pore dispersing agent and water, each having the amount of addition
shown in Table 3, were added to silicon carbide powder, and put into a
ball mill and mixed for 48 hours to make slurry. As a molding additive,
binder was added and mixed to the slurry, followed by spray drying,
thereby preparing silicon carbide granules having a mean particle
diameter of 80 μm.

[0142]As the above pore forming agent, suspension-polymerized
non-crosslinked resin beads composed of previously ground polystyrene
were used, which had a roundness of 4 μm or less and a diameter of 12
to 75 μm, and had the maximum diameter shown in Table 3. As the above
pore dispersing agent, polycarboxylic acid sodium was used.

[0143]Subsequently, the mixed raw material was charged into a mold and
pressed and molded at a pressure of 98 MPa in the thickness direction,
thereby obtaining a molded body having a predetermined shape.

[0144]The temperature of the obtained molded body was raised for 20 hours
in nitrogen atmosphere, and held at 600° C. for 5 hours, then
spontaneously cooled and defatted, thereby obtaining a defatted body. The
defatted body thus obtained was then held at 2000° C. for 4 hours
and sintered to obtain Sample Nos. III-1 to III-6, each being a sintered
body.

[0145]The surface of each sample was then ground to obtain a flat surface
by a surface grinding machine, and roughly worked by a lapping machine
made of alumina using diamond abrasive grains having a mean particle
diameter of 3 μm. Subsequently, the surface thereof was
mirror-finished by a lapping machine made of tin using the diamond
abrasive grains having a mean particle diameter of 3 μm, so as to have
an arithmetic mean height Ra of 0.98 μm or less, thereby obtaining a
sliding surface. The subphase shape in the sliding surface was observed
at 5000 times magnification by a Scanning Electron Microscope. No
columnar subphase was observed in each sample, and only the granular
subphase was observed.

[0146]The ratio of spherical pores, and the ratios of Si and C (atomic %)
in the primary phase and that of the subphase were measured in the same
manner as in Example 1. The measured values and the measuring results are
shown in Table 3. The porosity of each sample was found according to
Archimedean method.

[0147]The three-point bending strength, the Poisson's ratio, the Young's
modulus, the coefficient of thermal expansion at 40° C. to
400° C. and the thermal conductivity in each sample were measured
in the same method as described in Example II, and the coefficient of
thermal shock resistance R' defined by the equation (4) was found.

[0148]Separately, a ring-shaped molded body was manufactured and defatted
and sintered, thereby obtaining a sintered body. The surface thereof was
ground to obtain a flat surface by a surface grinding machine, and
roughly machined by a lapping machine made of alumina. Subsequently, the
surface thereof was mirror-finished by a lapping machine made of tin so
as to have an arithmetic mean height Ra of 0.98 μm or less, thereby
obtaining samples, each being an annular body having an outer diameter of
26 mm and an inner diameter of 19 mm. All of these samples were
stationary rings 5a.

[0149]Thereafter, they are slid under the same conditions as in Example
II, thereby measuring the coefficient of friction during sliding. The
measured values are shown in Table 3.

[0150]As apparent from Table 3, Sample No. III-1 having a porosity of less
than 2.5% is good because of a high thermal conductivity and a high
coefficient of thermal shock resistance R', however, the coefficient of
friction is high. Sample No. III-6 having a porosity exceeding 15% is
good because of a low coefficient of friction, however, both the thermal
conductivity and the coefficient of thermal shock resistance R' are low.

[0151]On the other hand, Samples No. III-2 to III-5 having a porosity of
2.5% to 12% are suitable because the thermal conductivity, the
coefficient of thermal shock resistance R' and the coefficient of
friction are well balanced.